Failed to add user extension while using gazelle

[jackylee-ch]

Describe the bug
When we start spark with user spark extension and gazelle, it will throw "Spark extensions are already specified before enabling Gazelle plugin: ".

To Reproduce
Start spark with use spark extension and gazelle, which means we would set spark.sql.extensions in spark-defaults.conf to another extension.

Expected behavior
This exception should only be thrown when user set spark.sql.extensions to GazelleSessionExtensions or ArrowWriteExtension.

Additional context
None

[PHILO-HE]

Assume it is already fixed. Please re-open it if necessary.